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This paper presents a comprehensive site survey of the Delingha site on the northern Tibetan Plateau from 2012 to 2014. In this campaign, we investigated 24 h continuous astronomical seeing, meteorological properties, and |$C_{n}^{2}$| in the ground layer (GL). The results show that the median seeing is 3.46 arcsec during the day and 1.80 arcsec during the night. The best nighttime seeing occurs in summer (1.66 arcsec), while the worst occurs in autumn (1.96 arcsec). Seeing exhibits obvious diurnal behaviour, with the worst seeing occurring around mid-day (11:00 to 14:00, Beijing time), and the best seeing just before sunset (18:00 to 20:00, Beijing time). Moreover, the Delingha site has good meteorological properties for astronomical observations during the measurement period, especially the low median wind speed (2.8 m s−1) and great temperature inversion (day: −0.3 K, night: 0.6 K). As for the vertical distribution of |$C_{n}^{2}$|⁠ , the exponential h −0.46 dependence of |$C_{n}^{2}$| varying with height in the GL is also obtained, and its power index is significantly related to the seasons. Furthermore, we calculated the contribution of optical turbulence in the GL to the whole atmosphere. Turbulence within the GL contributes significantly in summer, and its contribution can reach more than 35 per cent. Finally, we present the complete dependence analysis of seeing on meteorological parameters at the Delingha site. The results show that the best seeing tends to occur when the temperature inversion is sufficiently large, but the dependence of seeing on wind speed is not significant. [ABSTRACT FROM AUTHOR]

In this paper, we present comprehensive night-time seeing statistics at the Lenghu site using data collected between 2018 and 2021 using a differential image motion monitor. The results show that the median seeing is 0.80 arcsec, which slightly larger than that previously reported due to climate change. Additionally, we investigated the annual, seasonal, and monthly seeing statistics, as well as possible hourly seeing trends in each season. The results demonstrate that the seeing at the Lenghu site is very stable, with monthly median seeing varying from 0.6 to 0.95 arcsec. The seasonal dependence of seeing shows obvious variations over different years. The best year of seeing was 2019 (0.74 arcsec), and the best seeing occurred in spring (0.68 arcsec) to early summer (June, 0.63 arcsec), while the worst seeing occurred in winter (0.80 arcsec). The minimum seeing value throughout the night is at approximately three hours before dawn. Furthermore, wind speed profiles, especially V 200 (particularly the wind speed at 200 mbar), are important parameters for evaluating astronomical sites in terms of their suitability for adaptive optics. We first show the complete monthly average behaviours of the wind speed profile and V 200 using the European Center for Medium-Range Weather Forecast reanalysis data sets. The results demonstrate the annual periodicity and variability of V 200, and the yearly V 200 modulation function is also provided. V 200 maintains large values throughout the autumn (average: 35.01 m s−1) and winter (average: 34.33 m s−1), while the minimum V 200 occurs in spring (average: 27.93 m s−1). [ABSTRACT FROM AUTHOR]

Appropriate knowledge of wind-speed distributions and optical turbulence at existing and potential astronomical observatories is crucial for siting ground-based telescopes and applying adaptive optics (AO) systems. In this paper, the wind-speed and optical-turbulence characteristics above Gaomeigu and the Tibetan Plateau are studied by employing the 20-yr (1999–2018) European Centre for Medium-Range Weather Forecasts' fifth set of reanalysis data (ERA5). First, the meteorological parameters derived from ERA5 data are evaluated with coinciding radiosonde measurements. Results show that the meteorological parameters of ERA5 data in the free atmosphere have quite good reliability, with bias and root mean square error basically lower than 1.2 K in temperature and basically smaller than 2 m s−1 for wind speed. Then, vertical distributions and seasonal behaviour of the wind speed at Gaomeigu and Lhasa station above the Tibetan Plateau are analysed. Thirdly, the Richardson number (Ri) in the free atmosphere is calculated to provides us with a map of relative probability of different periods and regions of optical turbulence being developed above the two sites. In general, the atmospheric stability of Gaomeigu is higher than that of Lhasa station. Particularly in June, for Gaomeigu, the atmospheric stability within 6–30 km a.s.l. is basically superior or equal to the stable condition found at two mid-latitude sites: Oukaimeden and La Palma. Moreover, Lhasa station has a relative higher stability during June–September than other months. Furthermore, we provide the |$({C_n^2})$| profiles using ERA5 data at Gaomeigu and Lhasa. The results indicate that the choice of an appropriate outer-scale model is crucial for revealing local turbulence characteristics. [ABSTRACT FROM AUTHOR]